Bank of America  has escalated a longstanding dispute with MBIA over securities backed by residential mortgages.

The nation's second-biggest bank by assets accused MBIA on Thursday of defaulting on some of its debt when the insurer changed the indentures governing the obligations.

In November, MBIA obtained the consent of holders of the debt to the changes, which may have lowered leverage Bank of America holds over MBIA by virtue of the bank's refusal to yield to MBIA's demand that it repurchase allegedly defective securities issued by Countrywide, the mortgage lender Bank of America acquired in 2008.

Bank of America responded with a tender offer to buy the debt, to prevent what the bank said was a risk regulators would seize MBIA if the changes succeeded.

In a statement Thursday, Bank of America accused MBIA of defaulting on the debt "through, among other things, the purported adoption of a proposed amendment in violation of the terms of the indenture."

In an email, MBIA spokesman Kevin Brown termed Bank of America's allegation "meritless" and an attempt by the bank to force MBIA "to accept a discounted settlement of the over $4.5 billion that B of A owes MBIA for fraudulent and misrepresented mortgage loans, to the detriment of MBIA's policyholders and its other stakeholders."

A Bank of America spokesman declined to comment.

MBIA insured principal and interest payments on mortgage-backed securities issued by Countrywide. The insurer has sued Bank of America to recover billions of dollars MBIA paid to investors who bought the securities, which dropped significantly in value.

Bank of America said Thursday it had purchased roughly $136 million of the notes MBIA sought to change, via the bank's tender offer.

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